You are using MediaStudio Pro? If so you see that the audio is located on an audio track below the video track. You need to first split the audio from the video. This can be done using the right-click menu, split audio. Once that is done, select the audio, and press the delete key. Now you just drag your new audio file to the audio track of your choice.

If you're using VideoStudio, right-click on the video, select split audio. Now the audio will be located on one of the audio tracks. Select it and press the delete key. Then drag your new (mp3) audio to the music or narrative track.